Crookston Castle sits atop a natural hill, emphasised by the early ring-ditch, which can still be seen. To the north is a steep drop to the Levern Water. The castle has a rectangular main block, which was strengthened by a tower at each corner. This formed an irregular 'X-plan' shape, an unusual layout also seen at Hermitage Castle.[7] Only the north-east corner tower survives to its former height, as well as the basement of the south-east tower. ( Wikipedia)

And if you get the opportunity you should go and see the new exhibition by Aboyne Academy which has just opened at Braemar Castle: http://www.deesidepiper.co.uk/news/local-headlines/castle-is-home-to-new-exhibition-1-2374309

Bothwell Castle is a large medieval castle sited on a high, steep bank, above a bend in the River Clyde, it is Scotland’s largest and finest 13th century castle. Part of the original circular keep survives. The 13th-century donjon – without parallel in Scotland. The 13th-century prison tower – a good example of the grimness of medieval prison life. The 14th-century great hall and chapel – impressive survivals from the period of the Black Douglases.

I am very lucky to have four castles very close to where I live, Bothwell, Cadzow, Craignethan and Strathaven. The photo attached to this comment is of Strathaven. The first castle on this site was built in 13th cent, todays ruins are from the castle built in the 16th cent.